In the process control industry, it is known that many process applications require control valves that leak very small amounts of a process fluid into the surrounding environment. In fact, some process plants are subject to federal regulation under the 1990 Amendments to the Clean Air Act which regulates the amount of certain process emissions, such as aromatic or chlorinated hydrocarbons, based upon measured emission concentrations (e.g., less than 500 parts per million by volume (ppmv)) that leak from control valve assemblies into the plant environment. Typical solutions to reduce such emissions include placing a bellows seal around the control valve stem to contain the emissions or installing spring-loaded or live-loaded packing assemblies within the control valve body to maintain the emissions at acceptable concentration levels during valve operation.
Typical bellows seals create an external, “accordion-like” environmental seal by attaching a flexible metal chamber (i.e., a bellows) around an exposed portion of the valve stem. The bellows seals are intended to capture and contain process fluids within the bellows chamber, thereby preventing escape to the surrounding environment. To be functional, the bellows must remain flexible through a large operational temperature range and be resistant to various types of corrosion, which generally requires the use of special metals. Bellows are generally made from expensive alloys such as Inconel® from Special Metals Corporation of New Hartford, N.Y. or Hastelloy® C from Haynes International, Inc. of Kokomo, Ind. Both special metals significantly increase the cost of the bellows seal. Additionally, bellows seals are expensive to install as the bellows are generally seal-welded to the valve stem, gasket-sealed at the bonnet/valve joint and require an extended valve bonnet. The physical construction of the bellows and this installation method also places limits on the amount of rotation that can occur in the valve stem. In order to prevent damaging the weld or the seal, an anti-rotation device must often be installed to limit the amount of valve stem rotation during operation. Bellows seals are also designed for a specific length of travel to maximize bellows fatigue life. Applications producing travel greater than the designed length of travel may damage the bellows by extending the “folds” beyond the designed length causing premature cycle fatigue or cracking to occur. An alternative to capturing the leaking emissions in a bellows seal is to prevent the emissions from occurring using improVed control valve packing such as live-loaded packing.
Conventional live-loaded packing sets are installed within a packing bore of the control valve assembly to seal around the valve stem to substantially reduce emissions from the packing set during operation. It is generally understood that the packing must be axially loaded or stressed to force radial expansion of the packing components to affect a dynamic seal on a moving valve stem and a static seal in the packing bore where the packing components are in contact within the control valve body. As used in the present description, it should be understood by one of ordinary skill in the art that the term packing stress means an axial force from a loading device, such as a spring, or from process pressure acting on the packing set that is divided by the annular area of the packing. Furthermore, the packing assemblies described herein use V-ring sealing components (i.e., the cross-section of the packing is in the shape of a “V”) designed to amplify the axial packing stress into a larger radial contact stress to promote sealing by concentrating the axial forces in radial directions. It is generally known that environmental, live-loaded packing assemblies have certain limitations. FIG. 1 graphically represents the various types of example packing stress relative to a process packing pressure, A, described in detail below. One skilled of ordinary skill in the art should appreciate that packing stresses below the process pressure, A, may generally result in process fluid leaks since the process pressure may overwhelm a seal formed by the packing stress.
One type of conventional live-loaded packing is termed automatic packing. A seal is provided by a single V-ring packing set that is axially loaded by a coil spring that exerts a relatively small packing stress on the packing rings such as Single PTFE packing available from Fisher Controls International LLC of St. Louis, Mo. One skilled in the art understands that this type of packing set uses a V-ring with a high axial force-to-radial force ratio. That is, the V-ring is constructed to provide high radial expansion under the relatively low spring rate of a coil spring for a given application. This type of automatic packing is typically rated for environmental service (e.g., <500 ppmv concentration) at a maximum pressure of 300 psi, as shown in FIG. 1 as axial packing stress B, and a maximum temperature of 200° F. These types of packing may be loaded from the inboard or pressure side of the control valve, but are generally only applicable to low pressure, environmental applications due to the coil spring loading.
Another type of packing is generally described as double V-ring packing. This packing assembly uses two low pressure V-ring packing sets similar to the single V-ring packing described above with the packing sets arranged as an upper and a lower seal component, but without any type of spring loaded device to exert the packing stress. The packing set is stressed under a static packing load to create the valve stem seal with a packing nut/packing follower assembly known to those skilled in the art. The shortcoming of is type of packing is that without a spring element to ensure an adequate level of packing stress over a large temperature range, the packing design cannot be rated for environmental service, and, as such, is not depicted in FIG. 1.
Yet another type of environmental packing is a double V-ring, live-loaded packing set commercially available as Enviro-Seal® PTFE packing from Fisher Controls International LLC of St. Louis, Mo. This type of packing set uses a high pressure V-ring (i.e. a low ratio of axial force-to-radial force) loaded by a high-spring rate loading device such as a Belleville spring. In comparison to coil spring loading, the Belleville springs have a much greater spring rate to provide a relatively large force or packing stress required to compress the double V-ring packing for high pressure applications. This type of packing is typically rated for environmental service at a maximum pressure of 750 psi and a maximum temperature of 450° F. One issue with this type of packing assembly relates to the uses of Belleville springs to load the packing. Although the Belleville springs provide the required packing stress, the travel or range of compression of the Belleville springs is quite low. This combination of high spring rate and low or small travel range results in the need for very precise initial adjustment of the Belleville spring preload and/or tightly held manufacturing tolerances to obtain the desired packing stress. That is, one of ordinary skill in the art should appreciate that the packing stress per unit travel or compression of the Belleville springs is relatively large. As such, normal manufacturing tolerances within the control valve assembly necessitate manual adjustment, which can be very difficult and time consuming (e.g., the Fisher Controls Design D2 dump valves uses three Belleville spring stacked in series, which require adjustment precision within ±0.0024 inches to achieve a packing stress within ±50 psi). Thus, if the packing stress is too high, high packing friction may result, which can reduce control valve performance and packing life.
Additionally, coil springs typically are not used with high pressure, double V-ring packing due to the fact that bonnet/packing box area is limited and the cross-sectional area of coil spring needed to develop the proper spring rate will be too large. Furthermore, this type of packing set is typically loaded from the outboard side (i.e. external or atmospheric side as compared to the inboard or pressure side) of the control valve providing a packing force that opposes a force produced by the process pressure. Because the Belleville spring force opposes the force produced by the process pressure, the spring forces are not additive to the packing stress; therefore, the initial packing stress required to create the environmental seal must be accounted for in the initial packing setup by increasing the initial packing stress, as shown as a packing stress C of FIG. 1, which is independent of the process pressure until the process pressure matches the packing stress. This overcompensation in the initial packing stress creates greater friction in the assembly, which may cause the control valve actuator to be oversized, thereby adding expense to the control valve and resulting in greater packing wear during operation.
Another commercially available packing suited for high-temperature, high-pressure environmental service is a graphite-based packing with integrated PTFE known as Enviro-Seal Graphite ULF from Fisher Controls International LLC of St. Louis, Mo. This type of packing set uses graphite-based packing rings for high temperature operation with small amounts of PTFE integrated in seal components to minimize friction. Belleville springs are used to supply the packing stress. Unlike the previous packing sets, the extremely high axial force-to-radial force ratio of the graphite-based seal rings requires very high spring rates to create the environmental seal. For this type of packing, the Belleville springs create a very large force from the opposite direction of a force generated by the process pressure resulting in a packing stress that can approximate 4500 psi (shown as constant packing stress D in FIG. 1). Similar to other types of Belleville spring-based packing assemblies, the travel of the Belleville springs is very low requiring very precise initial adjustments to control the packing stress. Although this type of packing is rated for environmental service at a maximum pressure of 1500 psi and a maximum temperature of 600° F., the friction levels produced by this packing arrangement may be substantially higher than PTFE packing at temperatures below 300° F. and may be unacceptable in certain types of applications (e.g., applications without control valve positioners).
Accordingly, it is desired to provide an improved live-loaded packing system with improved operating range of performance which can apply a uniform stress to the valve stem packing, such that the packing stress remains at a constant level above a process pressure during operation. It is also desired to provide a live-loaded packing system to reduce packing friction for improved control valve performance and reduced packing wear for improved maintenance.